Influence of hydrogen on softened heat-affected zones during in-situ slow strain rate testing in advanced high-strength steel welds

Hydrogen Stress Cracking Behaviour in Dissimilar Welded Joints of Duplex Stainless Steel and Carbon Steel

“…This strain explains the dimple fracture observed in the HSC fractography (Figure 7b). To explain the effect of hydrogen on this phenomenon, a hydrogen-enhanced strain-induced void (HESIV) mechanism was suggested, which was observed in our previous study [17]. According to the HESIV theory, voids can be easily formed by the hydrogen at the intersections of hydrogen trapping sites such as carbide interfaces, lath boundaries, and dislocations [17,36,37].…”

“…After polishing the gauge section up to #2400 grit SiC paper, hydrogen was electrochemically pre-charged with a current density of 50 A/m 2 for 48 h in 3% NaCl and 0.3% NH 4 SCN aqueous solution containing As 2 O 3 (0.5 g L −1 ) to prevent the recombination of hydrogen atoms at the specimen surface. On the basis of our previous study, we assumed that the time required for hydrogen to diffuse through the ferrite phase to the centre of the tensile specimen with a thickness of 2 mm would be approximately 5 h [17]. Therefore, considering the difference between the hydrogen charging conditions used in this study and those used in the previous study, pre-charging for 48 h was determined to be sufficient to reach the equilibrium distribution up to the ferrite phase and the interface of the ferrite and austenite phases within the gauge length.…”

“…The SSRT was conducted at a cross-head speed of 2.5 × 10 −5 mm s −1 (nominal strain rate = 10 −6 s −1 ). The schematic of the in situ SSRT is shown in the authors' previous study [17].…”

“…To evaluate the HSC sensitivity of the WJs, their ΔRoAs were measured instead of quantifying the decrease in their displacement. This is because the transverse WJs had a non-uniform microstructure distribution within the gauge section [17]. The ΔRoAs of the WJs with the DF and AF were almost the same (~65%), indicating that hydrogen caused rapid fracturing immediately after reaching the ultimate tensile stress (UTS) with little plastic deformation.…”

“…Low carbon steel normally has good weldability, and the welding of high strength steels produces a softened zone where the strain is concentrated making it vulnerable to HSC initiation. Furthermore, the welding microstructures have high hardness, such as MA constituent and martensite, accelerate hydrogen embrittlement [16][17][18]. In the case of dissimilar-welded joints in DSS and carbon steel, the ferrite phase fraction is increased in the heat affected zone (HAZ) of the DSS and the softened HAZ of high-strength carbon steels.…”